The present invention relates to an apparatus and a method for controlling the combustion in stratified charge combustion engines. More particularly, the present invention pertains to an apparatus and a method for controlling the combustion in stratified charge combustion engines that are provided with brake boosters, which use vacuum pressure to improve braking force.
In a typical engine, fuel is injected into an intake port from a fuel injector. The fuel is then homogeneously mixed with air and sent into a combustion chamber through the intake port. An engine performing homogeneous charge combustion has a throttle valve arranged in its intake manifold. The size of the throttle valve opening is adjusted by manipulating an acceleration pedal to vary the amount of air supplied to the combustion chamber. This ultimately changes the amount of homogeneously mixed air and fuel. Engine power is controlled, accordingly.
However, homogeneous charge combustion produces strong vacuum in the intake manifold when the throttle valve throttles the intake manifold. The vacuum increases energy loss due to pumping, or pumping loss, which occurs when the air-fuel mixture is drawn into the combustion chamber through the intake port.
On the other hand, an engine capable of performing stratified charge combustion has been proposed. Stratified charge combustion is performed by injecting fuel directly into the combustion chamber such that the fuel is concentrated about the spark plug. Ignition by the spark plug burns the stratum of rich air-fuel mixture surrounding the spark plug. This, in turn, burns the stratum of lean air-fuel mixture surrounding the rich mixture. The power of a stratified charge combustion engine is basically controlled by adjusting the amount of fuel injected toward the vicinity of the spark plug. Accordingly, pumping loss is decreased since the throttle valve does not throttle the intake manifold. This improves the efficiency of the engine. Furthermore, stratified charge combustion improves fuel efficiency since the engine can be driven with a lean air-fuel mixture.
Japanese Unexamined Patent Publication No. 8-164840 describes an engine that performs stratified charge combustion. In this engine, each combustion chamber is provided with a homogeneous charge fuel injection valve and a stratified charge fuel injection valve. The homogeneous charge injection valve homogeneously disperses fuel into the combustion chamber and the stratified charge injection valve injects fuel toward the vicinity of the ignition plug. When the engine load is small, fuel is injected from the stratified charge injection valve. Thus, the fuel is supplied in a concentrated manner about the ignition plug. In this sate, stratified charge combustion is performed with the throttle valve opened slightly wider than when homogeneous charge combustion is performed.
This engine is also provided with a brake booster that increases the braking force thereby decreasing the required force for depressing the brake pedal. The brake booster uses vacuum, which is produced in the intake manifold downstream of the throttle valve, as a drive source. Specifically, the brake booster is connected to the downstream side of the throttle valve by a pipe. The pipe communicates vacuum to the brake booster. The communicated vacuum acts on a diaphragm incorporated in the brake booster and increases the force actuating the brake. The force of the communicated vacuum corresponds to the degree of depression of the brake pedal.
However, in a stratified charge combustion engine, the throttle valve is normally not throttled during stratified charge combustion. Therefore, the pressure in the intake manifold is not lowered, and there is insufficient vacuum available for braking. In the engine of the above publication, the opening of the throttle valve is decreased if there is not sufficient vacuum for actuating the brake booster during stratified charge combustion. As a result, the pressure in the intake manifold is lowered and sufficient vacuum is produced to actuate the brake booster.
Decreasing the opening of the throttle valve during stratified charge combustion decreases the amount of air supplied to the combustion chamber compared to decreasing the valve opening during normal stratified charge combustion. The decreased intake air amount enriches the air-fuel mixture in the vicinity of the spark plug. If the amount of injected fuel is increased due to increased engine load in this state, the mixture in the vicinity of the spark plug becomes too rich with less fuel compared to a case where the throttle valve opening is not decreased. This destabilizes fuel combustion and increases the specific fuel consumption of the engine.